Gordon Brown launched a scathing personal attack on the Liberal Democrat leader Nick Clegg yesterday, ridiculing his policies and likening him to a TV gameshow host. This final attempt to prevent Labour slumping to a humiliating third place in Thursday's general election was made in an exclusive interview with the Observer.

With polls predicting that Labour will lose its 13-year grip on power – and Clegg claiming the election was "a two-horse race" between his party and the Tories – Brown appeared desperate to prick the Lib Dem bubble.

In a withering assault, during which he repeatedly laughed out loud at Lib Dem policies, Brown said Clegg could have cobbled together his proposals on a night out with friends.

"I mean, you know, when you actually look at their economic policy, these regional caps on immigration, this amnesty for illegal immigrants, this tax policy that's built on £4.5bn coming from tax avoidance," he said, "it's the sort of stuff that you do when you're at a dinner party looking at your policies and writing them on the back of an envelope."

He suggested the surge of support for the Lib Dem leader and his party was based merely on his smoothness in front of the television camera.

"We're talking about the future of our country. We're not talking about who's going to be the next presenter of a TV gameshow. We're talking about the future of our economy."

Brown admitted that Labour was now the "underdog" in the race for No 10 but said he believed that with four days to go he still had time turn things round and convince the record number of undecided voters. "This is the biggest decision for a generation," he said.

The prime minister added that the "brutal truth" was that Labour had to win the election to safeguard the economic recovery and protect hundreds of thousands of jobs from being lost as a result of Tory cuts.

A batch of new polls last night showed the Tories increasing their lead while Labour and the Lib Dems battle it out for second place. However, with a hung parliament still on the cards, Brown refused to be drawn on any discussion about coalition deals. But he hinted that he would not try to cling on to power if his party fell into third place. "Of course I respect the view of the electorate," he said. "I am a democrat."

Brown said he intended to fight for the remainder of the campaign on the issue of jobs and which party had the best policies for safeguarding the economic recovery.

He described himself as a "serious man" with "serious policies" who had shown he could lead the country through the toughest times and got the big decisions on the economic crisis right.

In terms that will infuriate the Conservatives, the prime minister said David Cameron and his party knew that their economic policies would destroy countless jobs. "There would be hundreds of thousands of jobs at risk under the Conservative government. I mean there's no doubt, if the Conservatives are in power, unemployment will be higher. I don't think even they would disagree with that."

A Tory emergency budget – promised within 50 days of the election if they win – would wreck the recovery, the prime minister claimed.

"They would take away tax credits from a million people. So, they take away people's consuming power, their spending power. You take money out of the economy; that means that people have less to spend.

"It means that people buy less. It means that retail is affected. It means that all sorts of things happen as a result of that. Then you take away £6bn from the public services and that means teachers' jobs, it means police jobs."

Yesterday, while touring a farmers' market in his Oxfordshire constituency of Witney, Cameron said his party now had the "momentum" behind it and that he faced the final days of the campaign with "energy, energy, energy".

He dismissed talks of a deal with the Lib Dems, saying an outright Tory victory was vital. "That is good for the country, it needs strong leadership in uncertain times."